Cross-linked epoxy resins with non-linear optical properties

ABSTRACT

The epoxy resins according to the invention demonstrate the following structure: ##STR1## The cross-linked epoxy resins exhibit non-linear optical properties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to new epoxy resins and their use.

2. Description of Related Art

Non-linear optics deals with the interaction of the electromagneticfield of a light wave spreading in a medium with this medium, as well aswith the related occurrence of new fields with changed properties.Specifically, if the electromagnetic field enters into interaction withthe medium, which consists of one molecule or of many molecules, thenthis field polarizes the molecules.

Polarization, which is induced by a local electrical field in amolecule, can be represented as the power series of the electrical fieldintensity--corresponding to Equation (1):

    P=α.E+β.E.sup.2 +γ.E.sup.3 + . . .        (1);

P is the induced polarization and E is the induced local electricalfield, and α, β and γ represent the polarizability of the first, secondand third order.

On a macroscopic level, a similar relation holds true--according toEquation (2)--for polarization induced, by an external electrical field,in a medium consisting of several molecules:

    P=ε.sub.o (χ.sup.(1).E+χ.sup.(2).E.sup.2 +χ.sup.(3).E.sup.3 +. . . )                           (2);

again, P is the induced polarization and E is the induced localelectrical field, ε_(o) is the dielectric constant, and χ.sup.(1),χ.sup.(2) and χ.sup.(3) represent the dielectric susceptibility of thefirst, second and third order.

The dielectric susceptibilities in Equation (2) have a meaning similarto that of the molecular coefficients in Equation (1): They are materialconstants, which are dependent on the molecular structure and thefrequency, and, in general also on the temperature. The coefficientsχ.sup.(2) and χ.sup.(3) cause a great number of non-linear opticaleffects, specifically depending on the input frequency and the distanceof the molecular oscillation frequencies or electronic resonances, andthe input frequencies or frequency combinations, as well as the phaseadaptation conditions.

Materials with a dielectric susceptibility of the second order aresuited for frequency doubling (SHG=Second Harmonic Generation); this isthe transformation of light with a frequency ω into light with afrequency 2ω. Another non-linear optical effect of the second order isthe linear electro-optical effect (Pockels Effect); it results from thechange in the index of refraction of the optical medium when anelectrical field is applied. Optical rectification as well as sum anddifference frequency mixing are further examples of non-linear opticaleffects of the second order.

Areas of use for materials of the type stated above are, for example,electro-optical switches as well as areas of data processing andintegrated optics, such as optical chip-to-chip connections,wave-guiding in electro-optical layers, Mach Zehnder interferometers andoptical signal processing in sensor technology.

Materials with a dielectric susceptibility of the third order are suitedfor frequency tripling of the incident light wave. Additional effects ofthe third order are optical bistability and phase conjugation. Concreteapplication examples are purely optical switches for constructing purelyoptical computers and holographic data processing.

To achieve a sufficient non-linear optical effect of the second order,the dielectric susceptibility of the second order χ.sup.(2) must begreater than 10⁻⁹ electrostatic units (esu); this means that thehyperpolarizability β must be greater than 10⁻³⁰ esu. Anotherfundamental prerequisite for achieving a non-linear optical effect ofthe second order is the non-centrosymmetrical orientation of themolecules in the non-linear optical medium; otherwise, χ.sup.(2) =0.This can be achieved with an orientation of the molecular dipoles, if itis not predetermined by the crystal structure, as in the case ofcrystalline materials. Thus, the greatest values for χ.sup.(2) for anon-linear optical medium have been achieved by orientation of themolecular dipoles in electrical fields.

Inorganic materials, such as lithium niobate (LiNbO₃) and potassiumdihydrogen phosphate (KH₂ PO₄), demonstrate non-linear opticalproperties. Semiconductor materials, such as gallium arsenide (GaAs),gallium phosphide (GaP) and indium antimonide (InSb), also demonstratenon-linear optical properties.

However, along with the advantage of a high electro-optical coefficientof the second order, inorganic materials of the type stated have somemajor disadvantages. For example, the processing of these materials isvery complicated in terms of technology, since individual process stepsare time-consuming and must be carried out with extremely high accuracy(see in this regard: C. Flytzanis and J. L. Oudar "Nonlinear Optics:Materials and Devices," Springer-Verlag (1986), pages 2 to 30). Thesematerials are furthermore unsuitable for those electro-opticalcomponents which work at high modulation frequencies. Due to the highdielectric constants which intrinsically present, the dielectric losseswhich occur at high frequencies (above several GHz) are so high thatworking at these frequencies is impossible (see in this regard: "J. Opt.Soc. Am. B," Vol. 6 (1989), pages 685 to 692).

It is known that organic and polymer materials with extended π electronsystems, which are substituted with electron donors and acceptors,demonstrate non-linear optical properties, i.e. can be used innon-linear optical media (see in this regard: R. A. Hann and D. Bloor"Organic Materials for Non-linear Optics," The Royal Society ofChemistry (1989), pages 382 to 389 and 404 to 411).

Monocrystals on an organic basis demonstrate a high electro-opticalcoefficient of the second order and good photochemical stability, incomparison with LiNbO₃ ; the required high level of orientation of thenon-linear optical molecules is also already present. Some significantcriteria, however, speak against technical utilization of this materialclass. For example, production of the monocrystals, specifically bothfrom solution and from a melt, requires a time of 14 to 30 days (see inthis regard: D. S. Chemla and J. Zyss "Nonlinear Optical Properties ofOrganic Molecules and Crystals," Academic Press, Inc. (1987), Vol. 1,pages 297 to 356); the production process therefore does not meet therequirements of technical production. Furthermore, the melting point ofthe crystals lies at 100° C., on the average, so that it would not bepossible to achieve a working temperature range up to 90° C.Furthermore, organic crystals cannot be structured and their lateraldimensions are presently still too small to allow them to be constructedas an electro-optical component.

For applications of non-linear optics in the areas of data transmissionand integrated optics, polymer materials have found increasingimportance recently. For this purpose, an external electrical field isapplied to polymer materials heated above the glass transitiontemperature; this results in orientation of the non-linear opticalmolecules. After cooling (below the glass transition temperature), withthe electrical field applied, anisotropic and thereforenon-centrosymmetrical polymers are obtained, which demonstratedielectric susceptibilities of the second order.

Non-linear optical compounds which are dissolved or diffused in polymerscan be processed to form thin layers, in this manner, as is required byintegrated optics (see in this regard: "Macromolecules," Vol. 15 (1982),pages 1385 to 1389; "Appl. Phys. Lett.," Vol. 49 (1986), pages 248 to250; "Electron. Lett.," Vol. 23 (1987), pages 700 and 701). However, thelow solubility of the compounds with a low molecular weight, theirinsufficient distribution in the polymers, the migration of the activemolecules out of the polymer matrix and the loss of thenon-centrosymmetrical orientation of the active molecule species over aperiod of only a few hours, even at room temperature, aredisadvantageous in this connection.

Polymers with covalently bonded non-linear optical molecule components,which simultaneously have a liquid-crystalline character, are also knownas non-linear optical compounds (see in this regard: EP-OS 0 231 770 andEP-OS 0 262 680). While these materials do not demonstrate thedisadvantages stated above, they are not suited for applications inelectro-optics and integrated optics in their current stage ofdevelopment, since optical losses >20 dB/cm, caused by the inherentdomain scattering, occur here. Furthermore, studies of amorphousnon-linear optical polymers have already been reported (see:"Macromolecules," Vol. 21 (1988), pages 2899 to 2901).

Both with liquid-crystalline polymers and with amorphous polymers withcovalently bonded non-linear optical molecule units, a highconcentration of such units can be achieved. A spacer thereby uncouplesthe molecular mobility of the non-linear linear optical units from thepolymer chain; at the same time, however, the glass transitiontemperature decreases drastically. However, with this, a loss of themolecular orientation of the non-linear optical molecule units and aloss of the non-linear optical activity must be expected at usetemperatures in the range of the glass transition temperature of thepolymers.

SUMMARY OF THE INVENTION

It is an object of the invention to expand the available supply ofpolymers for non-linear optical media and, in particular, to makepolymers available which demonstrate a technically sufficient glasstransition temperature. This is accomplished, according to theinvention, with epoxy resins with the following structure: ##STR2##where the following applies: x:y=1:99 to 99:1;

R¹ and R² =H, CH₃ or halogen;

X¹ and X² =O or NR³, with R³ =H or alkyl (linear or branched) with 1 to6 C atoms;

Y¹ =alkylene (linear or branched) with 2 to 20 C atoms, where one ormore non-adjacent CH₂ groups, with the exception of the binding CH₂group to the group Z, can be replaced with O, S or NR⁴ (R⁴ =H or C₁ toC₆ alkyl);

Y² =alkylene (linear or branched) with 1 to 3 C atoms;

Z is a conjugated π electron system (E) with the structure -D-E-A,substituted with an electron donor (D) and an electron acceptor (A),where the following applies: ##STR3## with m=to 1 to 3,

E¹ =--(CH═CH)_(n) --, --N═N--, --CH═N--, --N═CH-- or --C.tbd.C--, withn=1 to 3, and

E² =CH or N;

D=O, S, NR⁵, PR⁶ or NR⁷ --NR⁸, with R⁵, R⁶, R⁷ and R⁸ =hydrogen, alkyl,alkenyl, aryl or heteroaryl; and

A=halogen, NO, NO₂, CN, CF₃, COR⁹, COOR¹⁰, SO₂ OR¹¹, SO₂ NR₂ ¹²,##STR4## with R⁹, R¹⁰, R¹¹ and R¹² =hydrogen, alkyl, alkenyl, aryl orheteroaryl;

where X¹, Y¹ and D or Y¹ and D together can also form a heterocyclicgroup containing nitrogen.

The groups X¹, Y¹ and D together, for example, can form a piperazinering, the groups Y¹ and D can form a piperidine or pyrrolidine ring.

Preferably, the following applies:

R¹ =R² =CH₃,

X¹ =X² =O,

Y¹ =(CH₂)_(o), with o=2 to 6,

Y² =CH₂, and

Z=-D-E-A with D=O or NR⁵, A=NO₂ or ##STR5## and

E= ##STR6##

In the epoxy resins according to the invention, the ratio of x:y isadvantageously 1:9 to 9:1; preferably, this ratio is 3:7 to 7:3.

DETAILED DESCRIPTION OF THE INVENTION

The conjugated π electron system (E) can be substituted with at leastone more electron donor on the electron donor side, in addition to D,and at least one more electron acceptor on the electron acceptor side,in addition to A. These substituents should be selected in such a waythat the total of the Hammet constants σ of the additional substituentsin each case does not exceed the value of the existing substituents (Dor A).

The invention therefore consists of new epoxy resins of the type statedabove. Furthermore, the invention consists of the use of these epoxyresins in cross-linked form, for non-linear optical media, i.e. fornon-linear optical polymers in the form of cross-linked epoxy resinswith the preceding structure.

It was surprisingly found that cross-linked epoxy resins of the typedescribed, with covalently bonded non-linear optical molecule units, onthe one hand do not demonstrate the disadvantages stated above, but onthe other hand do possess the known good polymer-specific properties,such as the ability to be processed to form thin layers, in the μmrange, high concentration of non-linear optical molecule units, lowoptical attenuation and technically sufficient glass transitiontemperature. This is due to the fact that in the epoxy resins accordingto the invention, structuring of the non-linear optical polymer layer toform wave-guide structure can take place by cross-linking, somethingthat is not possible with non-cross-linked polymers.

The production of non-linear optical polymers by cross-linking accordingto chemical methods is already known (see in this regard: "J. Appl.Phys.," Vol. 66 (1989), pages 3241 to 3247). For this purpose, solubleprepolymers are first formed by reaction of bisphenol A diglycidyl etherwith 4-nitro-1.2-phenylene diamine, which are then converted toinsoluble cross-linked polymers by being heated. In correspondingmanner, non-linear optical polymers can also be produced fromN,N-diglycidyl-4-nitroaniline and N-(2-aminophenyl)-4-nitroaniline (seein this regard: "Appl. Phys. Lett.," Vol. 56 (1990), pages 2610 to2612). Furthermore, electro-optical polymer films which are obtainedfrom reaction of 4-nitroaniline with bisphenol A diglycidyl ether areknown from "J. Opt. Soc. Am. B," Vol. 7 (1990), pages 1239 to 1250.

The epoxy resins according to the invention are amorphous copolymers,which are composed of comonomers which demonstrate covalently bondednon-linear optical molecule units, i.e. functional groups which causecross-linking. The production of the epoxy resins by radicalpolymerization, as well as the synthesis of the prestages, takes placeaccording to known methods (cf. in this regard the examples).

Radical polymerization can take place both by means of radicalinitiators which decompose thermally, and by means of initiators whichdecompose under the effect of light with a shorter wave length.Azoisobutyric acid nitrile and per compounds, such as dibenzoylperoxide, are preferably used as radical initiators which decomposethermally. Preferred photochemical initiators used are Michler's ketone,i.e. 4,4'-bis(dimethylamino)-benzophenone and its derivatives, as wellas anthraquinones, xanthones, acridinones, benzoines, dibenzosuberonesand onium salts, such as diaryl iodonium or triaryl sulfonium salts.

Cross-linking of the epoxy resins according to the invention can takeplace either thermally or photochemically.

For thermal cross-linking, reagents which cause cross-linking are addedto the epoxy resins according to the invention, in a corresponding molarratio; in general, these are compounds with acidic hydrogen atoms,substances from which such compounds can be produced, or compounds withelectrophilic groupings. Cross-linking then takes place at a raisedtemperature, preferably at a temperature which lies 15° C. above theglass transition temperature of the cross-linked end product, i.e. ofthe polymer. If necessary, the cross-linking reaction can be catalyzedwith the addition of an accelerator. The compounds or substances usedfor thermal cross-linking are known. Preferably, carboxylic acidanhydrides, phenols and aliphatic, cycloaliphatic or aromatic amines areused for this.

Suitable carboxylic acid anhydrides are, in particular, phthalic acidanhydride, tetrahydrophthalic, hexahydrophthalic,methyltetrahydrophthalic and endomethylene tetrahydrophthalic acidanhydride, pyromellithic acid, trimellithic acid and benzophenonetetracarboxylic acid anhydride, as well as maleic acid anhydride/styrenecopolymers. Amines that can be used are, in particular,4.4'-diaminodiphenyl methane, as well as its o,o'-alkyl substitutedderivatives and hydrogenated 4.4'-diaminodiphenyl methane,4.4'-diaminodiphenyl ether, 4.4'-diaminodiphenyl sulfone,2.4-diamino-3.5-diethyl toluene, isophoron diamine, diethylene triamine,triethylene tetramine and polyaminoamides on the basis of diethylenetriamine.

Photochemical cross-linking of the epoxy resins according to theinvention takes place using light with a shorter wave length, preferablylight in the UV spectrum (290 to 390 nm). To trigger photochemicalcross-linking, initiators are added, which release Lewis or Bronstedacids under the influence of light; such compounds are known.Preferably, aryl diazonium, diaryl iodonium or triaryl sulfonium salts,which have tetrafluoroborate, hexafluorophosphate orhexafluoroantimonate as the anion, as well as arene iron salts, are usedas initiators.

To improve the surface quality, the processability and/or thecompatibility with polymers, processing aids can be added to the epoxyresins, depending on their purpose of use. These are, for example,thixotropic agents, flow agents, plasticizers, cross-linking agents,lubricants and binders.

The epoxy resins according to the invention are applied to a substratein dissolved or liquid form, if necessary together with compounds thatcause cross-linking, or initiators, by centrifugation, dipping, printingor spreading. In this manner, a non-linear optical arrangement isobtained, where the epoxy resins or corresponding prepolymers areoriented in dipolar manner in electrical fields, before and/or aftercross-linking. After cooling, polymer materials with excellentnon-linear optical properties are obtained, and, due to thecross-linking, they have increased orientation stability and thusgreater long-term stability, even at higher use temperatures.

To produce the non-linear optical media, it is particularly advantageousto use oligomer prepolymers, with a low molecular weight, of the epoxyresins according to the invention. The production of these prepolymerstakes place in known manner, where the epoxy resins are brought toreaction with a shortage of the compound which causes cross-linking.After application to a substrate, the prepolymers are oriented indipolar manner--above the glass transition temperature--and subsequentlycross-linked to yield the non-linear optical polymers (with an improvedproperty profile).

On the basis of embodiments, the invention will be explained in greaterdetail below.

EXAMPLE 1 Production of 4-benzoyloxybiphenyl

128 ml benzoyl chloride is dripped into a solution of 170 g4-hydroxybiphenyl in 500 ml pyridine, at 20° C.; subsequently, thereaction mixture is heated under reflux for 30 min. After cooling, thereaction mixture is mixed with a solution of 500 ml concentratedhydrochloric acid and 750 ml water. The remaining precipitate isfiltered off, washed neutral with water, dried and recrystallized fromn-butanol (melting point: 150° C.); yield: 94%.

EXAMPLE 2 Production of 4-benzoyloxy-4'-nitrobiphenyl

A mixture of 274 g 4-benzoyloxybiphenyl (see Example 1) and 2000 mlglacial acetic acid is heated to 85° C. Then, 650 ml fuming nitric acidis dripped in such a manner that the temperature does not drop below 90°C. After cooling, the precipitate is filtered off, washed neutral withwater and recrystallized from glacial acetic acid (melting point: 214°C.); yield: 60%.

EXAMPLE 3 Production of 4-hydroxy-4'-nitrobiphenyl

The equivalent amount of a 40% caustic soda solution is added, by drops,to a mixture of 300 g 4-benzoyloxy-4'-nitrobiphenyl (see Example 2) and1500 ml ethanol; after addition, the mixture is heated under reflux for30 min and then allowed to stand at room temperature overnight.Subsequently, the sodium salt is filtered off and dried. The free base,i.e. the hydroxy compound, is released by suspension of the sodium saltin water and acidification with half-concentrated hydrochloric acid Theraw product is filtered off, washed neutral with water andrecrystallized from ethanol (melting point: 204° C.); yield: 80%.

EXAMPLE 4 Production of 4-(6-hydroxyhexyloxy)-4'-nitrobiphenyl

A solution of 71 g potassium hydroxide in 300 ml water is dripped underreflux, into a mixture of 215 g 4-hydroxy-4'-nitrobiphenyl (see Example3) and 4000 ml ethanol. Then, 217 g 6-bromohexanol is added,subsequently, the mixture is heated under reflux for 15 h. Aftercooling, the solvent is removed in a vacuum and the residue is slurriedup with water dried. The raw product is filtered off, washed neutralwith water and recrystallized from ethanol (melting point: 119° C.);yield: 65%.

EXAMPLE 5 Production of 4-(6-methacryloxyhexyloxy)-4'-nitrobiphenyl

77 g 4-(6-hydroxyhexyloxy)-4'-nitrobiphenyl (see Example 4) is dissolvedin 1500 ml absolute dioxane, and the solution is then heated to 45° C.Subsequently, 26.8 g triethylamine and 27.2 g methacrylic acid chloride,dissolved in the same volume of dioxane, are dripped in, then thereaction mixture is stirred for 24 h. Subsequently, the solvent isremoved in a vacuum and the reaction mixture is suspended in water; thesolid is filtered off, washed with water, dried and recrystallized fromethanol (melting point: 57° C.); yield: 80%.

EXAMPLE 6 Production of a copolymer

For copolymerization of 4-(6-methacryloxyhexyloxy)-4'-nitrobiphenyl (seeExample 5) with methacrylic acid glycidyl ester, these two componentsare brought to reaction, together with 1 mole-% azoisobutyric acidnitrile, in 20 ml absolute, oxygen-free toluene, in a Schlenk tube, at70° C., under nitrogen; the reaction period is 18 h. The raw productobtained is dissolved in trichloromethane and then precipitated inmethanol. The results obtained are summarized in Table 1 (T_(G) =glasstransition temperature).

                  TABLE 1                                                         ______________________________________                                        Polymer  Composition x:y Yield   T.sub.G                                      No.      in mole %       in %    in °C.                                ______________________________________                                        P 1      75:25           80      93                                           P 2      50:50           90      86                                           ______________________________________                                    

EXAMPLE 7

For cross-linking, the copolymers according to Example 6--with theaddition of 1 mole-% N,N'-dimethylbenzylamine--are mixed with carboxylicacid anhydrides and hardened at a raised temperature.

The results obtained are summarized in Table 2, where the followingapplies:

HHPSA=hexahydrophthalic acid anhydride

PSA=phthalic acid anhydride

                  TABLE 2                                                         ______________________________________                                        Polymer Composition    Hardening tem-                                                                             T.sub.G                                   No.     in mole %      perature in °C.                                                                     in °C.                             ______________________________________                                        P 1.1   100 P 1:100 HHPSA                                                                            150          145                                       P 2.1   100 P 2:100 HHPSA                                                                            150          130                                       P 1.2   100 P 1:100 PSA                                                                              170          155                                       P 2.2   100 P 2:100 PSA                                                                              170          145                                       ______________________________________                                    

EXAMPLE 8

For the electro-optical studies, the epoxy resins according to theinvention or corresponding prepolymers, if necessary together withcompounds which cause cross-linking, in a suitable solvent, are appliedto ITO-coated glass (ITO=indium-tin-oxide) by spin-coating; the layerthickness of the films produced in this manner is usually 3 to 6 μm. Forelectrical poling, to achieve a high level of non-centrosymmetricalorientation, a gold electrode is sputtered onto the film (of the epoxyresin); the counter-electrode is the light-permeable ITO layer. Afterheating of the sample to the glass transition temperature range, aconstant voltage is applied, where the required voltage increase isadjusted to the orientation behavior of the non-linear optical moleculeunits, in order to avoid electrical punctures and therefore adestruction of the film. After a poling field intensity of 50 to 100V/μm has been reached, a poling period of 15 min is sufficient fororientation of the non-linear optical molecule units.

Subsequently, the sample is cross-linked, either thermally(corresponding to Example 7) or photochemically, and then the sample iscooled to room temperature, with a constant electrical field applied toit, causing the orientation to become fixed.

Electro-optical examination of the polymer samples takes place byinterferometric measurement of a laser beam applied at an angle, aftersingle reflection at the gold electrode. The measurement setup requiredfor this, and the measurement evaluation, are known (see, for example:"Appl. Phys Lett.," Vol. 56 (1990), pages 1734 to 1736). Theelectro-optical coefficients r₃₃ of the polymers according to Example 7,i.e. of the cross-linked epoxy resins, which relate to a poling fieldintensity of 70 V/μm, are summarized in Table 3.

                  TABLE 3                                                         ______________________________________                                                      Electro-optical co-                                             Polymer No.   efficient in pm/V                                               ______________________________________                                        P 1.1         2.5                                                             P 2.1         2.7                                                             P 1.2         1.2                                                             P 2.2         1.3                                                             ______________________________________                                    

What is claimed is:
 1. An epoxy resin of the structure ##STR7## wherethe following applies: x:y=1:99 to 99:1;R¹ and R² =H, CH₃ or halogen; X¹and X² =O or NR³, with R³ =H or alkyl (linear or branched) with 1 to 6 Catoms; Y¹ =alkylene (linear or branched) with 2 to 20 C atoms, where oneor more non-adjacent CH₂ groups, with the exception of the binding CH₂group to the group Z, can be replaced with O, S or NR⁴ (R⁴ =H or C₁ toC₆ alkyl); Y² =alkylene (linear or branched) with 1 to 3 C atoms; Z is aconjugated π electron system (E) with the structure -D-E-A, substitutedwith an electron donor (D) and an electron acceptor (A), where thefollowing applies: E= ##STR8## with m=1 to 3,E¹ =--(CH═CH)--_(n),--N═N--, --CH═N--, --N═CH-- or --C.tbd.C--, with n=1 to 3, and E² =CH orN; D=O, S, NR⁵, PR⁶ or NR⁷ --NR⁸, with R⁵, R⁶, R⁷ and R⁸ =hydrogen,alkyl, alkenyl, aryl or heteroaryl; and A=halogen, NO, NO₂, CN, CF₃,COR⁹, COOR¹⁰, SO₂ OR¹¹, SO₂ NR₂ ¹², ##STR9## with R⁹, R¹⁰, R¹¹ and R¹²=hydrogen, alkyl, alkenyl, aryl or heteroaryl;where X¹, Y¹ and D or Y¹and D together can also form a heterocyclic group containing nitrogen.2. The epoxy resin according to claim 1, wherein the followingapplies:R¹ =R² =CH₃, X¹ =X² =O, Y¹ =(CH₂)_(o), with o=2 to 6, Y² =CH₂,and Z=-D-E-A with D=O or NR⁵, A=NO₂ or ##STR10## and E= ##STR11##
 3. Theepoxy resin according to claim 2 wherein the ratio of x:y is from 1:9 to9:1.
 4. The epoxy resin according to claim 3 in cross-linked form. 5.The epoxy resin according to claim 2 wherein the ratio of x:y is from3:7 to 7:3.
 6. The epoxy resin according to claim 5 in cross-linkedform.
 7. The epoxy resin according to claim 2 in cross-linked form. 8.The epoxy resin according to claim 1 wherein the ratio of x:y is from1:9 to 9:1.
 9. The epoxy resin according to claim 8 in cross-linkedform.
 10. The epoxy resin according to claim 1 wherein the ratio of x:yis from 3:7 to 7:3.
 11. The epoxy resin according to claim 10 incross-linked form.
 12. The epoxy resin according to claim 1 incross-linked form.